A spectrophotometer used in an analyzer (such as a liquid chromatograph) extracts only a desired wavelength component from a spectrum of light emitted from a light source, illuminates a sample component with the extracted light, detects transmitted light, and thereby measures absorbance. A deuterium lamp, a tungsten halogen lamp, or the like is used as the light source. The deuterium lamp mainly emits light in the ultraviolet region, while the tungsten halogen lamp emits light in the visible region.
To light a deuterium lamp, a negative electrode is first heated by a heater or the like to emit thermoelectrons. In this state, a voltage (trigger voltage) is applied between the positive electrode and negative electrode to initiate an electric discharge of deuterium gas existing between the positive electrode and negative electrode (initial discharge). Furthermore, when the initial discharge grows while the trigger voltage is being applied, the impedance between the positive electrode and negative electrode begins to decrease, triggering a main discharge.
A constant-current power supply that operates when a load impedance is at or below a predetermined threshold level is connected between the positive electrode and negative electrode. When the impedance between the positive electrode and negative electrode falls to the threshold as a result of the main discharge, the constant-current power supply comes into operation to cause a predetermined current to flow, maintaining the main discharge and turning on the lamp (see Patent Document 1).
FIG. 3 shows a typical power supply circuit used to light the deuterium lamp. The power supply circuit 20a is roughly divided into three parts: a heater power supply 21, a trigger power supply 22a, and a constant-current power supply 23. The heater power supply 21 is used to supply an electric current to the negative electrode 26 and thereby heat the negative electrode 26, while the trigger power supply 22a is used to produce an initial discharge. The constant-current power supply 23 is used to maintain a main discharge after a transition from the initial discharge. Normally, one end of the deuterium lamp 24a on the side of the negative electrode 26 is grounded.
To light the deuterium lamp 24a, an electric current is first supplied to the negative electrode 26 (filament) from the heater power supply 21 (a variable voltage source) to heat the negative electrode (filament) 26 and thereby cause the filament 26 to emit thermoelectrons. In the trigger power supply 22a, a three-terminal switch S21 is set to the side of a constant-voltage power supply E21, and a capacitor C21 is charged until its voltage becomes equal to that of the constant-voltage power supply E21 (normally on the order of 400 to 600 V).
Next, the switch S21 is set to the side of the positive electrode 25 of the deuterium lamp 24a and the voltage of the capacitor C21 is applied between the positive electrode 25 and negative electrode 26 via a resistor R21. The applied voltage causes an initial discharge, which further grows into a main discharge. As a result of the main discharge, the impedance between the positive electrode 25 and negative electrode 26 falls, causing a constant current (around 300 mA) to flow from the constant-current power supply 23, thereby maintaining the main discharge and turning on the lamp.
Various switches are available including a mechanical switch (mechanical relay) and a semiconductor switch, but in the circuit configuration shown in FIG. 3, it is difficult to use a semiconductor switch, because a high switch-to-ground voltage on the order of 400 to 600 V is applied to the switch S21 placed between the positive electrode 25 and capacitor C21. Under such a condition, it is necessary to use a mechanical switch with a superior resistance to high voltages.
The discharge characteristics of the deuterium lamp deteriorate with age due to wear and tear of the electrodes and consumption of deuterium gas. Therefore, even if a constant trigger voltage is applied between the positive electrode and negative electrode, the initial discharge may not be able to grow in the previously described manner.
Thus, to ensure that an electric discharge will more reliably start, a deuterium lamp equipped with an auxiliary electrode between the positive electrode and negative electrode has been developed. In this deuterium lamp, the distance between the auxiliary electrode and negative electrode is configured to be shorter than the distance between the positive electrode and negative electrode. Consequently, when a voltage is applied between the auxiliary electrode and negative electrode, an initial discharge is produced relatively easily, and if a voltage is applied between the positive electrode and negative electrode at the same time, the initial discharge between the auxiliary electrode and negative electrode will serve as a pilot light in causing the initial discharge between the positive electrode and negative electrode to grow easily into a main discharge.
FIG. 4 shows a typical power supply circuit used to light a deuterium lamp provided with an auxiliary electrode. The deuterium lamp 24b differs from that of FIG. 3 in that the deuterium lamp 24b is equipped with an auxiliary electrode 27 as well as with a capacitor C22, resistor R22, and switch S22 used to apply a voltage between the auxiliary electrode 27 and negative electrode 26.
To turn on the deuterium lamp 24b, not only the capacitor C21, but also the capacitor C22 are charged in advance by the constant-voltage power supply E21 via the switch S22. Then, by simultaneously setting the switches S21 and S22 to the side of the positive electrode 25 of the deuterium lamp 24b, the voltage of the capacitor C22 is applied between the auxiliary electrode 27 and negative electrode 26 via the resistor R22 and at the same time the voltage of the capacitor C21 is applied between the positive electrode 25 and negative electrode 26 via the resistor R21. Consequently, an initial discharge occurs due to the application of the voltage between the auxiliary electrode 27 and negative electrode 26 and grows into a main discharge due to the simultaneous application of the voltage between the positive electrode 25 and negative electrode 26. In this way, the deuterium lamp 24b is turned on.
As described so far, the voltage needed for the deuterium lamp to begin electric discharge is applied via a capacitor. By the application of the voltage, the capacitor discharges and the capacitor voltage falls sharply. Consequently, the voltage needed for the initial discharge to grow is applied for a short period of time; the time constant of a typical circuit configuration for the electric discharge is only a few μsec to a few tens of μsec. Therefore, it is important to time the voltage application between the positive electrode and negative electrode with the voltage application between the auxiliary electrode and negative electrode.
In the configuration of the power supply circuit of FIG. 4, in order to time the applications of the two voltages with each other, it is important to synchronize the two switches S21 and S22 with each other.